A digital image is comprised of a multitude of small picture elements or pixels. When a color digital image is rendered on a display device, a single pixel may be formed from red, green, and blue (RGB) sub-pixels. The sub-pixels in some RGB display devices may include either a red, green, or blue filter. The sub-pixels in a display device are spatially close and, for this reason, human vision perceives the red, green, and blue sub-pixels as a single-colored pixel. By modulating the colors of the individual sub-pixels, a range of colors can be generated for each pixel.
A color filter array (CFA) describes the arrangement of sub-pixels in color image sensors and in color display devices. A variety of CFAs are known. The Bayer CFA is one well-known example. Red, green, and blue sub-pixels are arranged in a square gird in the Bayer CFA. There are as many green sub-pixels as blue and red sub-pixels combined, with a green sub-pixel at every other position in both the horizontal and vertical directions, and the remaining positions being populated with blue and red sub-pixels. In the Bayer CFA, a single pixel includes two green and one each of blue and red sub-pixels.
Conventionally, the data for a color pixel define how much color each sub-pixel adds to the perceived color of the pixel. The data for each sub-pixel can vary within a range depending on the number of data bits allocated in the display system for sub-pixel values. For example, for 24-bit RGB color, 8 bits are allocated per sub-pixel, providing a range of 256 possible values for each color channel. If the data values for all components of an RGB pixel are zero, the pixel theoretically appears black. On the other hand, if all three sub-pixel values are at their maximum value, the pixel theoretically appears white. RGB pixel data expressed using 24-bits (8:8:8) provides for a color palette of 16,777,216 colors. Color pixel data, however, need not be expressed using 24-bits. RGB pixel data may be represented using as few as one bit per channel (1:1:1), providing a color palette of eight colors.
An electro-optic material has at least two “display states,” the states differing in at least one optical property. An electro-optic material may be changed from one state to another by applying an electric field across the material. The optical property may or may not be perceptible to the human eye, and may include optical transmission, reflectance, or luminescence. For example, the optical property may be a perceptible color or shade of gray.
Electro-optic displays include the rotating bichromal member, electrochromic medium, electro-wetting, and particle-based electrophoretic types. Electrophoretic display devices (“EPD”), sometimes referred to as “electronic paper” devices, may employ one of several different types of electro-optic technologies. Particle-based electrophoretic media include a fluid, which may be either a liquid, or a gaseous fluid. Various types of particle-based EPD devices include those using encapsulated electrophoretic, polymer-dispersed electrophoretic, and microcellular media. Another electro-optic display type similar to EPDs is the dielectrophoretic display.
An electro-optic display device may have display pixels or sub-pixels that have multiple stable display states. Display devices in this category are capable of displaying (a) two or more display states, and (b) the display states are considered stable. The display pixels or sub-pixels of a bistable display may have first and second stable display states. The first and second display states differ in at least one optical property, such as a perceptible color or shade of gray. For example, in the first display state, the display pixel may appear black and in the second display state, the display pixel may appear white. The display pixels or sub-pixels of a display device having multiple stable display states may have three or more stable display states, each of the display states differing in at least one optical property, e.g., light, medium, and dark shades of a particular color. For example, the display pixels or sub-pixels may display states corresponding with 4, 8, 16, 32, or 64 different shades of gray.
With respect to capability (b), the display states may be considered to be stable, according to one definition, if the persistence of the display state with respect to display pixel drive time is sufficiently large. An exemplary electro-optic display pixel or sub-pixel may include a layer of electro-optic material situated between a common electrode and a pixel electrode. The display state of the display pixel or sub-pixel may be changed by driving a drive pulse (typically a voltage pulse) on one of the electrodes until the desired appearance is obtained. Alternatively, the display state of a display pixel or sub-pixel may be changed by driving a series of pulses on the electrode. In either case, the display pixel or sub-pixel exhibits a new display state at the conclusion of the drive time. If the new display state persists for at least several times the duration of the drive time, the new display state may be considered stable. Generally, in the art, the display states of display pixels of liquid crystal displays (“LCD”) and CRTs are not considered to be stable, whereas electrophoretic displays, for example, are considered stable.
The level of reflectance of an electro-optic pixel or sub-pixel may be less than one hundred percent. Consequently, when a color image is rendered on an electro-optic display device, colors may tend to lack brightness. One technique for increasing brightness involves reducing the size of color filters associated with a display pixel. However, one problem with this technique is that it tends to reduce color saturation.
A digital image may be defined using more display states than an electro-optic display device may be capable of rendering. Thus, in order to render a digital image on an electro-optic display device, it may be necessary to reduce the bit-per-pixel resolution of the digital image. A color processing algorithm may pre-process a digital image before the image is to be rendered on an electro-optic display device. If the color processing algorithm processes the digital image in its native or original resolution, the algorithm may provide precision that will not be fully observable when the image is rendered on the electro-optic display device. Consequently, a color processing algorithm that only operates on image data in its native resolution may be wasteful of power and processing time.